1. Field of the Invention
The present invention relates to a process and apparatus for the production of an optical fiber preform used for the production of an optical fiber.
2. Description of the Related Art
A single mode optical fiber, for example, has a core of 10 .mu.m at its center and a cladding of an outer diameter of 125 .mu.m formed around the same. Such an optical fiber is produced by heating a porous optical fiber preform having a portion to become the core and a portion to become the cladding and drawing the same to the above-mentioned thickness (or diameter).
As processes for production of a porous optical fiber preform, there are known, for example, the outer vapor deposition method (OVD method) and the vapor axial deposition method (VAD method).
Here, a description will be made of the process of production of a porous optical fiber preform common to the OVD method and VAD method. First, a starting preform comprised of the portion to become the core of the optical fiber or the portion to become the core with a portion to become part of the cladding formed around it is produced. This starting preform is glassified to produce a seed rod (or a starting rod). The seed rod is rotated as a target. An oxygen-hydrogen burner is used to hydrolyze vapor of a feedstock gas such as SiCl.sub.4 in the oxygen-hydrogen flame to form particulates including glass particulates such as SiO.sub.2. The particulates are blown from the oxygen-hydrogen burner to the rotating target to adhere to and deposit on the soot body to form the cladding on the target. The particulates are continued to be deposited on the deposited soot body from the oxygen-hydrogen burner until the soot body reaches a predetermined outer diameter. The seed rod with the predetermined outer diameter of a soot body formed on it is used as the porous optical fiber preform.
The deposition yield of glass (SiO.sub.2) particulates on the target by this process of production of an optical fiber preform is about 20 to 40 percent (%) at most and the deposition speed is about 5 to 10 g/minute, so various processes have been proposed aimed at improving the deposition efficiency, improving the production efficiency, and reducing the production costs. Various processes have also been proposed for improving the quality of the porous optical fiber preform.
Japanese Unexamined Published Patent Application No. 58-161936 discloses a process for production of an optical fiber preform by the OVD method wherein electrodes are laid in the seed rod along the axial center of the same, the seed rod or the soot body formed around the outside of the same is charged with a negative polarity, and the resultant static electricity is used to improve the deposition rate of the glass particulates on the target. This process, however, has the disadvantage that the holes made for laying the electrodes along the axial center of the seed rod remain in the optical fiber after the optical fiber preform is formed, heated, and drawn to produce the optical fiber. Further, in this process, when the outer diameter of the soot body becomes larger, the soot body is charged from inside the seed rod, so when the outer diameter of the soot body becomes larger, the charge on the surface of the soot body falls and there is the problem that an optical fiber preform with a large outer diameter cannot be efficiently produced.
Japanese Unexamined Published Patent Application No. 57-67038, Japanese Unexamined Published Patent Application No. 58-217447, Japanese Unexamined Published Patent Application No. 58-217448, and Japanese Unexamined Published Patent Application No. 60-36341 disclose the OVD and VAD methods of the production of optical fiber preforms, similar to the above method. In particular, Japanese Unexamined Published Patent Application No. 58-217447 discloses the VAD method wherein a high DC voltage is applied to a flow of gas to charge the gas flow with negative ions, the negative ion gas is blown on the surface of the target to charge the target with a negative polarity, and the potential difference is used to cause glass particulates to deposit on the target. Even with these processes, however, it is still not possible to achieve the desired improvement of the deposition yield of glass particulates and the shortening of the production time.
The process disclosed in Japanese Unexamined Published Patent Application No. 58-217447 further suffers from the problem explained below. When glass particulates and ion gas with an opposite polarity are blown on a target, the flow of the glass particulates heading toward the target is obstructed by the flow of the ion gas, so the deposition (yield) of glass particulates on the target falls. Ideally, the opposite polarity ion gas is supposed to cause the surface of the target to be charged to a polarity opposite to the glass particulates so that the glass particulates are drawn to the surface of the target where they are electrically neutralized, but in practice a considerable portion ends up neutralized in the space between the burner and the target and the effects hoped for cannot all be obtained.